A conventional method of providing for fault tolerant transmission through a transmission device is through the use of standby facilities which are not in use during no fault conditions, that is, during normal operation. More specifically, U.S. Pat. No. 4,497,054 to Read employs a standby digital switch which can be substituted for any one of a plurality of primary switch elements. During normal operation the standby digital switch remains idle.
Another conventional method of providing for fault tolerance through a transmission device is through the use of standby facilities which are used for the transmission of data under normal operations. More specifically, in U.S. Pat. No 4,455,645 to Mijioka et al, PCM signals are sent out on active and standby lines to a pair of switching modules. The active PCM line and the corresponding standby PCM line are never connected to the same switching module. When one of the switching modules fails, standby lines are then used to shift traffic to other working switching modules. The output active and standby lines, however, are not compared to each other for the detection on an erroneous output signal.
U.S. Pat. No. 3,920,914 to Regnier et al provides fault tolerance by providing multiple paths for a communication with each path going through a different and redundant switch. If during normal operation one of the paths is found inoperative, another path through the switch can be used. The signals on the output of each path, however, are not compared to each other for the detection of erroneous signals.
U.S. Pat. No. 4,276,637 to Le Dieu presents a design of a TDM switch with the feature wherein, when any one input module (terminal module in FIG. 2) fails, the traffic coming to the input module can still be properly switched through the switch. The approach proposed is to partition replicated traffic into other input modules. A so called management center will control the selection of the active path. Basically, Le Dieu's patent deals with the provision of the multiple paths, but it has nothing to do with the fault detection or correction using redundant paths.